Polymer nanocomposite foams

ABSTRACT

Nano-sized particles such as nano-clays can be mixed with polymers through either melt compounding or in-situ polymerization. By modifying the particle surface with various surfactants and controlling processing conditions, we are able to achieve either intercalated (partial dispersion) or exfoliated (full dispersion) nano-clay distribution in polymers with the clay content up to 35% by weight. When a blowing agent is injected into the nanocomposite in an extruder (a continuous mixer) or a batch mixer, polymeric foam can be produced. Supercritical carbon dioxide, an environmentally friendly, low-cost, non-flammable, chemically benign gas is used as the blowing agent. This process forms a microcellular foam with very high cell density (&gt;10 9  cells/cc) and small cell size (&lt;5 microns) can be achieved by controlling the CO 2  content, melt and die temperature, and pressure drop rate.

FEDERALLY SPONSORED RESEARCH

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Grant No.EEC-9815677 awarded by the National Science Foundation.

TECHNICAL FIELD OF THE INVENTION

The present invention is in the field of polymer foams. Specifically,the present invention relates to polymer nanocomposite foams.

BACKGROUND OF THE INVENTION

The present invention hereby incorporates by reference, application No.10/425,565, entitled “Clay Nanocomposites Prepared by In-situPolymerization”, filed on Apr. 29, 2002.

Foamed polymers are found in applications ranging from packaging,insulation, cushions, adsorbents, to scaffolds for tissue engineering.The basic principle of foaming is to mix a blowing agent (typically agas) into a polymer melt and induce a thermodynamic instability througha temperature or pressure change to nucleate bubbles of the blowingagent.

In this invention, supercritical CO₂ (the critical temperature T_(c):31° C. and the critical pressure P_(c): 73.8 bar or 1074 psi), apotential replacement of the traditional foaming agents (hydrocarbon orchlorofluorocarbon), was applied, The liquid-like solubility andgas-like diffusivity make it possible to dissolve sufficient CO₂ in apolymer quickly. CO₂ is low-cost, non-flammable, chemically benign, andenvironmentally friendly.

Recently, microcellular foams, characterized by cell sizes smaller than10 μm and cell density larger than 10⁹ cells/cm³, have drawn a greatdeal of attention and interest. It has been shown that by keeping thecell (or bubble) size uniformly less than 10 microns in diameter, onecan greatly reduce material usage without compromising mechanicalproperties because the bubbles are smaller than the preexisting flaws ina polymer matrix.

The field of polymer/clay nanocomposites has grown rapidly in the pastdecade. In this work, nano-sized particles, nanoclays, are applied tomodify the cellular foams in both batch and continuous extrusion foamingprocess. The results show that with the addition of a very small amountof nanoclay into the polymer matrix, the nanocomposites exhibitsubstantial increase in many physical properties, including mechanicalstrength (tensile modulus and strength, flexural modulus and strength),thermal stability, flame retardance, and barrier resistance. Smectiteclays, such as montmorillonite (MMT), are of particular interest becausethey have a high aspect ratio (lateral dimension ˜200-500 nm, thickness<1 nm) and a high surface area. However, clay is hydrophilic in natureand incompatible with most polymers. To increase the compatibility andmiscibility of clay in polymer, the clay surface is modified by anorganic surfactant, typically ammonium cations with long alkyl chains.

Two idealized polymer/clay structures are possible: intercalated andexfoliated. Exfoliation involves extensive polymer penetration todisrupt the clay crystallite (tactoids), and the individualnanometer-thick silicate platelets are dispersed in the polymer matrix.If there is only limited polymer chain insertion in the interlayerregion, and the interlayer spacing only expands to a certain extentwithout losing layer registry, then an intercalated nanocomposites isthen formed.

Polymer foam is another area subject to intensive research. It is widelyused for insulation, packaging, and structural applications, to name afew. Microcelluar foam, which is characterized by cell size in the rangeof 0.1˜10 μm, cell density in the range of 10⁹ to 10¹⁵ cells/cc,provides improved mechanical properties as well as increased thermalstability and lower thermal conductivity.

Cell nucleation and growth are two important factors controlling cellmorphology. Particles can serve as a nucleation agent to improveheterogeneous nucleation. Some inorganic nucleation agents, such astalc, silicon oxide, kaoline, etc., are widely used. A fine dispersionof these nucleation agents can promote formation of nucleation centerfor the gaseous phase. Although a detailed explanation of theheterogeneous nucleation mechanism is still not available, the size,shape, and distribution, and surface treatment of particles have greatinfluences on the nucleation efficiency. In this work, we developed anew polymer nanocomposite foam preparation technology to create polymerfoams with controlled cell structure. In addition, clay may furtherimprove the foam properties, e.g., mechanical and barrier properties, aswell as fire resistance.

SUMMARY OF THE INVENTION

The present invention includes polymeric nanocomposite foams and amethod for forming polymeric nanocomposite foams.

A method for forming a polymeric nanocomposite foam of the presentinvention comprises the steps of providing a mixture comprising: apolymer, an organophilic clay, and a blowing agent; and processing saidmixture so as to cause formation of cells, thereby forming a polymericnanocomposite foam.

Although any appropriate amount of blowing agent may be used, it ispreferred that the mixture comprises at least 1% by weight of theblowing agent. It is more preferred that the mixture comprise at least4% by weight of the blowing agent. It is most preferred that the mixturecomprises at least 7% by weight of said blowing agent.

Although any desired amount of organophilic clay may be used, it ispreferred that the mixture contain at least 0.5% by weight of theorganophilic clay. It is more preferred that the mixture comprise atleast 5% by weight of the organophilic clay. It is further preferredthat the mixture comprise at least 10% by weight of the organophilicclay. It is most preferred that the mixture comprises at least 20% byweight of the organophilic clay.

While any appropriate polymer may be used in forming the polymericnanocomposite foam, it is preferred that the polymer is selected fromthe group consisting of polystyrene, poly(methyl methacrylate),polypropylene, nylon, polyurethane, elastomers, and mixtures thereof.

It is preferred that the organophilic clay is dispersed throughout thepolymer such that a x-ray diffraction pattern produced from the mixtureis substantially devoid of an intercalation peak for producingexfoliated polymeric nanocomposite foams. It is preferred thatorganophilic clay is dispersed throughout the polymer such that a x-raydiffraction pattern produced from the mixture contains an intercalationpeak for producing intercalated polymeric nanocomposite foams.

It is preferred that the organophilic clay comprises: a smectite clay;and a compound having the formula:

wherein R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is a chemicalstructure having a terminal reactive double bond; R3 is an alkyl group;and R4 is an alkyl group.

It is most preferred that the compound have n=15, R3 as CH₃, R4 as CH₃,and R2 as:

While any appropriate clay may be used, it is preferred to use smectiteclay. It is more preferred that the smectite clay is selected from thegroup consisting of montmorillonite, hectorite, saponite, laponite,florohectorite, and beidellite.

The blowing gas may be any traditional blowing gas used in industry (forexample: freon, nitrogen or air). However, it is preferred that theblowing agent is a supercritical fluid. It is most preferred that theblowing agent is supercritical carbon dioxide.

Cell size can vary widely depending upon operating conditions, however,it is preferred that the polymeric nanocomposite foam has an averagecell size less than about 20 microns. It is additionally preferred thatthe polymeric nanocomposite foam has an average cell size greater thanabout 15 microns.

Cell density can vary widely depending on operating conditions, however,it is preferred that the polymeric nanocomposite foam has an averagecell density greater than about 1×10⁶ cells/cm³. It is more preferredthat the polymeric nanocomposite foam have an average cell densitygreater than about 1×10⁹ cells/cm³.

The polymer nanocomposite foam may be closed cell foam or open cellfoam.

A polymeric nanocomposite foam of the present invention comprises apolymeric portion; an organophilic clay, the organophilic clay isdispersed throughout the polymeric portion; and a plurality of cellsdispersed throughout the polymeric portion.

While any appropriate polymer may be used in the polymeric nanocompositefoam, it is preferred that the polymeric portion comprises a polymerselected from the group consisting of polystyrene, poly(methylmethacrylate), polypropylene, nylon, polyurethane, elastomers, andmixtures thereof.

It is preferred that the organophilic clay is dispersed throughout thepolymer such that a x-ray diffraction pattern produced from the mixtureis substantially devoid of an intercalation peak for exfoliatedpolymeric nanocomposite foams. It is preferred that organophilic clay isdispersed throughout the polymer such that a x-ray diffraction patternproduced from the mixture contains an intercalation peak forintercalated polymeric nanocomposite foams.

While any organophilic clay may be used, it is preferred that theorganophilic clay portion comprises: a smectite clay; and a compoundhaving the formula:

wherein R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is a chemicalstructure having a terminal reactive double bond; R3 is an alkyl group;and R4 is an alkyl group.

It is most preferred that the compound have n=15, R3 as CH₃, R4 as CH₃,and R2 as:

While any appropriate clay may be used, it is preferred to use smectiteclay. It is more preferred that the smectite clay is selected from thegroup consisting of montmorillonite, hectorite, saponite, laponite,florohectorite, and beidellite.

Cell size can vary widely depending upon operating conditions, however,it is preferred that the polymeric nanocomposite foam has an averagecell size less than about 20 microns. It is additionally preferred thatthe polymeric nanocomposite foam has an average cell size greater thanabout 15 microns.

Cell density can vary widely depending on operating conditions, however,it is preferred that the polymeric nanocomposite foam has an averagecell density greater than about 1×10⁶ cells/cm³. It is more preferredthat the polymeric nanocomposite foam has an average cell densitygreater than about 1×10⁹ cells/cm³.

The polymer nanocomposite foam may be closed cell foam or open cellfoam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the chemical structure of the surfactant2-methacryloyloxyethylhexadecyldimethyl ammonium bromide (MHAB).

FIG. 2 provides XRD patterns for various PS/clay nanocomposites.

FIG. 3 is a TEM micrograph of intercalated PS/20A nanocompositedemonstrating the large clay aggregates that are still present in thematrix.

FIG. 4 is a TEM micrograph of exfoliated PS/MHABS nanocomposite showinghow the tactoids have been completely delaminated and uniformlydispersed.

FIG. 5 provides XRD patterns for various PMMA/clay nanocomposites.

FIG. 6 is a SEM micrograph of polystyrene foam produced by a batchfoaming procedure.

FIG. 7 is a SEM micrograph of PS/5% 20A foam produced by a batch foamingprocedure.

FIG. 8 is a SEM micrograph of PS/5% MHABS foam produced by a batchfoaming procedure.

FIG. 9 compares cell size and cell density for PS and PS/claynanocomposite foams.

FIG. 10 is a SEM micrograph of PMMA foam produced by a batch foamingprocedure.

FIG. 11 is a SEM micrograph of PMMA/5% 20A foam produced by a batchfoaming procedure.

FIG. 12 is a SEM micrograph of PMMA/5% MHABS foam produced by a batchfoaming procedure.

FIG. 13 compares cell size and cell density for PMMA and PMMA/claynanocomposite foams.

FIG. 14 is a SEM micrograph of PS/talc filler foam produced byextrusion.

FIG. 15 is a SEM micrograph of PS/20A foam produced by extrusion.

FIG. 16 is a SEM micrograph of PS/MHABS foam produced by extrusion.

FIG. 17 is a SEM micrograph of PS foam by a batch foaming procedure.

FIG. 18 is a SEM micrograph of PS/1% MHABS foam by a batch foamingprocedure.

FIG. 19 is a SEM micrograph of PS/10% MHABS foam by a batch foamingprocedure.

FIG. 20 shows the effect of clay concentration on cell size and densitybased upon the concentration of MHABS by a batch foaming procedure.

FIG. 21 is a SEM micrograph of pure PS foam.

FIG. 22 is a SEM micrograph of PS/2.5-wt % 20A foam.

FIG. 23 is a SEM micrograph of PS/5-wt % 20A foam.

FIG. 24 is a SEM micrograph of PS/7.5-wt % 20A foam.

FIG. 25 shows the relationship between pressure drop rate and cell sizefor three different materials.

FIG. 26 shows the relationship between pressure drop rate and celldensity for three different materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In accordance with the foregoing summary, the following presents adetailed description of the preferred embodiments of the invention thatis currently considered to be the best mode.

Materials

Styrene (St), Methyl Methalcrylate (MMA) and initiator2,2′-azobisisobutyronitrile (AIBN), were purchased from Aldrich. Apolystyrene resin (Fina) was used to prepare nanocomposites byextrusion. Two types of organically modified montmorillonite clays wereused in this study. Cloisite 20A (20A) was donated by Southern Clay. Theinterlayer cation is dimethyl dehydrogenated tallowalkyl ammonium onium.Na⁺-montmorillonite (CEC=90 meq/100 g) was also from Southern Clay. Areactive cationic surfactant 2-methacryloyloxyethylhexadecylditnethylammonium bromide (MHAB) was synthesized and ion-exchanged withNa⁺-montmorillonite to prepare the organophilic clay, according to theprocedure published elsewhere. The chemical structure of the surfactantMHAB is shown in FIG. 1. The modified clay is denoted as MHABS. Praxairprovided the foaming agent, a bone-dry grade of carbon dioxide.

Preparation of Polymer/Clay Nanocomposites

Both twin-screw extrusion and in-situ polymerization was used to preparePS/clay and PMMA/clay nanocomposites. In-situ polymerization was carriedout under isothermal conditions at 60° C. for styrene and 50° C. forMMA. The monomer, clay and a certain amount of AIBN were mixed togetherusing a high shear mixer. The mixture was then reacted for 20 hrs, thenthe temperature was raised to 105° C. for another 30 min. 5% exfoliatednanocomposites prepared by in-situ polymerization were used forextrusion foaming. 20% exfoliated PS nanocomposite masterbatch wasblended with polystyrene (PS) to prepare exfoliated nanocomposites,using a DACA microcompounder for batch foaming. Intercalatednanocomposites were prepared using a Leistritz ZSE-27 intermesh twinscrew extruder (L/D=40, d=27 mm) operated in the co-rotating mode. Thescrew speed was 200 rpm.

Foaming of Polymer/Clay Nanocomposites

Batch foaming was performed at 120° C. CO₂ was delivered via a syringepump. The system was allowed to equilibrate for 24 hrs for CO₂ to reachsaturation in the polymer matrix. The pressure was then rapidly releasedand the foamed cells were fixed by cooling with water. The saturationpressure was 2000 psi and the pressure was released in 2-3 seconds forcell nucleation.

The microcellular foaming extrusion was performed on a two-stagesingle-screw extruder (HAAKE Rheomex 252P). A static mixer (Omega,FMX8441S) was attached to the end of the extruder to provide extramixing capacity. A capillary die with a 0.5 mm diameter and 10 mm lengthnozzle was custom made to generate a high and rapid pressure drop. CO₂was delivered from a syringe pump (ISCO 260D) with a cooling jacket. TheCO₂ pressure and volumetric flow rate can be read precisely from thepump controller.

CO₂ is compressed to a certain pressure in the syringe pump at 40° C.reaching a supercritical state. 4-wt % of CO₂ was injected into theextruder barrel by carefully controlling the pressure and volumetricflow rate of CO₂. Upon injection into the barrel, it is mixed with thepolystyrene melt by screw rotation. A single-phase solution is formedwhen the mixture flows through the static mixer. Nucleation occurs inthe die because of the solubility reduction due to the quick and largepressure drop realized by the narrow capillary nozzle. The foamedextrudate flows freely out to the air and vitrifies.

Analytical Methods

The X-ray diffraction (XRD) patterns of prepared polymer/claynanocomposites were recorded on a Scintag XDS-2000 X-ray diffractometerwith Cu Kα radiation and operated at 35 kV and 10 mA. Transmissionelectron microscopy (TEM) image was obtained from a Phillip CM12 usingan accelerating voltage of 80 kV. The nanocomposite samples weremicrotomed at room temperature with a diamond knife and mounted on a 200mesh copper grid. A Phillip XL30 scanning electron microscope was alsoused to observe the cell morphology.

Results and Discussion: Structure of Nanocomposites

Montmorillonite clay particles contain thousands of individual layerswith a thickness dimension ˜1 nm and lateral dimension ˜1 μm. Thepolymer chain penetration and interlayer expansion depend on thecompatibility of the polymer matrix and the clay surface. Intercalatednanocomposites usually form when there is only limited insertion of apolymer chain into the interlayer region. This results in the interlayerexpansion and can be detected by x-ray diffraction (XRD). FIG. 2 showsthe XRD of PS/clay nanocomposites. For PS/5% 20A, the shift of thediffraction peak to a lower angle region clearly verifies the polymerchain intercalation. The (d001) basal spacing increased from 2.3 nm to3.4 nm. The TEM micrograph of FIG. 3 demonstrates that large clayaggregates are still present in the matrix. Face-to-face layer stackingand low angle intergrowth of tactoids are still observable. On the otherhand, by using the reactive surfactant MHAB, the copolymerization ofMHAB and styrene monomer helped layer separation and exfoliatednanocomposite was synthesized with a clay concentration up to 20%. TheXRD of PS/5% MHABS and PS/20% MHABS show featureless pattern (FIG. 2). ATEM micrograph of PS/20% MHABS is shown in FIG. 4. The tactoids havebeen completely delaminated and uniformly dispersed in the matrix. Mostclay layers are present as single layers, while stacks of a few layersare also observable in some region. This nanocomposite was then blendedwith PS to make nanocomposites for the foam experiments. FIG. 5 showsthe XRD of PMMA/clay nanocomposites. For PMMA/5% 20A, the shift of thediffraction peak to a lower angle region clearly verifies the polymerchain intercalation. The (d001) basal spacing increased from 2.3 nm to3.6 nm. Again, the diffraction peak disappears for the exfoliatedPMMA/5% MHABS nanocomposite.

Effect of Clay Dispersion

Batch foaming experiments were conducted to compare the effect ofdifferent clay dispersions on the foam cell morphology, as shown inFIGS. 6-8. The clay concentration is 5-wt %. With the addition of clay,the cell size decreases and the cell density increases. Image analysiswas used to obtain the average cell size and cell density, and theresult is shown in FIG. 9. In the presence of 5% 20A, the cell sizedecreases from 20 μm to 15 μm, and the cell density increases from8.2×10⁷ to 1.3×10⁸ The exfoliated nanocomposite foam has an average cellsize of 11 μm and cell density of around 4.2×10⁸. The clay may serve asa heterogeneous nucleation agent allowing more sites to nucleate andgrow. This leads to an increase in cell density. While more cells startto grow at the same time, there is less opportunity for the individualcells to grow bigger, leading to a smaller cell size. In intercalatednanocomposites, most clay exists as stacks of layers or tactoids,serving as nucleation sites. On the other hand, in exfoliatednanocomposites, clay is present mostly as individual layers and usuallythe distance between the layers is greater than the effective radius ofgyration of a polymer chain. Unlike in intercalated nanocomposites wherepolymer chain penetration is limited and the major contact area is theouter surface of the tactoids, in exfoliated nanocomposites theindividual layer is in direct contact with the matrix, providing muchlarger interfacial area for CO₂ adsorption and cell nucleation. In otherwords, once exfoliated, the effective particle concentration is higherand the number of nucleation sites increases. As a result, theexfoliated nanocomposite foam shows the highest cell density and thesmallest cell size. Another factor that may affect the cell size anddensity is the rheological properties of the nanocomposites. Further, weobtained extremely small cell size (<1.6 μm) and large cell density(>10¹¹ cells/cm³) when PMMA and its nanocomposites were foamed even in abatch process. The morphology was shown in FIGS. 10-12. The major reasonis believed to be the higher solubility of CO₂ in PMMA. The cell sizeand cell density were compared in FIG. 13.

Both intercalated and exfoliated PS/clay nanocomposites were foamed in asingle screw extruder. For comparison, PS/talc foams were also preparedin the same extruder. The cell morphology is shown in FIGS. 14-16. Cellsin PS/talc are much larger and the cell density is much lower than thosein PS/20A at the same particle concentration. Once exfoliated, thenanocomposite foam has the smallest cell size and the highest celldensity. Exfoliated nanocomposite shows perfect microcellular foamstructure in which cells are round in shape, closed, and well separatedfrom each other. Very few cell coalescence was observed. The calculatedaverage cell size and cell density are 4.9 microns and 1.5×10⁹ cells/cm³respectively. In addition, the exfoliated composite extrudate exhibits avery smooth and shining surface that comes from the orderly alignment ofthe single clay layers, the small cell size, and the few flaws in thepolymer matrix.

Effect of Clay Concentration

A series of exfoliated PS/MHABS nanocomposites (1%, 5%, and 10%) werefoamed (T=120° C., P=2000 psi) to study the effect of clay concentrationon cell morphology. The SEM micrographs are shown in FIGS. 17-19. Andthe cell size and density are shown in FIG. 20. Adding 1% MHABS greatlyreduces the cell size and increases cell density. The cell size of thisnanocomposite foam is comparable to that of 5% 20A nanocomposite foam,while the cell density is higher. This supports our hypothesis that theindividual layers are capable of serving as nucleation sites. Eventhough the apparent concentration of MHABS is lower than 20A, there maybe more interfacial area between the clay and the matrix due toexfoliation. Adding 5% MHABS can further reduce cell size and increasecell density. However, further increasing the clay concentration to 10%seems only to increase cell density, while cell size remains almostunchanged. During foaming, both nucleation and growth will affect thecell size and density. And the cell growth depends strongly on thepolymer rheological properties, which are affected greatly by thepresence of clay. Both shear and elongational viscosity increase whenclay is added. The individual layers as well as the tactoids can form apercolated structure at the mesoscopic level, impeding the motion of thepolymer chain and thus increasing the shear viscosity. The exfoliatednanocomposites show a higher viscosity than the intercalatednanocomposites. The increase of the viscosity hinders the cell growth,resulting in a smaller cell size. The reason for the nearly the samecell size for the 5% and 10% nanocomposite foams is unclear. A possibleexplanation is as follows. When there are more clay platelets and morecell nucleation, It is more probable for cells to meet each other andform larger cells. This will lead to an increase in cell size.

We have showed that the addition of clay can help reduce cell size andincrease cell density. However, these nanocomposite foams are still arein the microcellular foam range. During batch foaming, the pressure droprate is not high enough, and therefore there is sufficient time forcells to grow. In the continuous extrusion foaming, the operatingconditions can be controlled to generate a high enough pressure drop. Infact, microcellular nanocomposite foams were prepared in our lab.

To investigate the effect of nanoclay on microcellular foamingextrusion, nanocomposites with different 20A concentration (0-10 wt %)were foamed in the single screw extruder under similar operationconditions. The cell size decreases dramatically after a small amount ofnanoclay (˜2.5 wt %) is blended in and then it levels off at high clayconcentration. However, the cell density increases nearly linearly.Exfoliated PS/MHABS nanocomposites with different compositions (0-20 wt.% MHABS) were also foamed. Similar trends in cell size and cell densitywere observed and more small cells were obtained compared withintercalated PS/20A nanocomposites.

Comparing the SEM images shown in FIGS. 21-24 of samples containing 0,2.5, 5 and 7.5 wt. % of 20A, it is found that the cells tend to coalescetogether when more 20A is blended in PS. This may be used to produceopen cell foams that are important for adsorption, filtration, andscaffolds of tissue engineering.

Effect of Operating Conditions

PS and PS/20A intercalated nanocomposites were also foamed at differentpressure drops by changing the screw rotation speed or the mass flowrate of the polymer/CO₂ mixture.

The results are summarized in FIGS. 25 and 26 that exhibit how cell sizeand cell density change with increasing pressure drop rate. Aninteresting phenomenon is that the decrease of the cell size becomesslower at high pressure drop rates, while the cell density increaseslinearly. Comparing with pure PS, nanocomposites make the microcellularfoaming process easier, where the cell size of nanocomposites can beeasily smaller than 10 μm and cell density larger than 10⁹ cells/cm³after the pressure drop rate is greater than 10⁹ Pa/sec. The exfoliatednanocomposite provides the smallest cells and largest cell density atthe lowest screw rotation speed (or the lowest pressure drop rate, 5×10⁸Pa/s).

Besides pressure, the influence of CO₂ concentration (0-8 wt %) andfoaming die temperature (120-240° C.) was also explored. Below the CO₂solubility limit, cell size decreases and cell density increases withthe increase of CO₂ concentration. A high CO₂ concentration is favorablefor producing open cell foams. Die temperature affects both cell sizeand cell structure (open or closed).

Comparing to conventional micron sized filler particles used asnucleation agents in the foaming process, the extremely fine dimensionsand large surface area of nanoparticles and the intimate contact betweenparticles and polymer matrix may greatly alter the cell nucleation andgrowth. It can absorb more CO₂ on its surface. The addition of nanoclayalso increases the viscosity of the polymer matrix. This may increasethe pressure drop rate in the die. The nanoclay can increase the celldensity and change the cell structure (open or closed). This becomesmore prominent when a polymer having low foaming ability withsupercritical CO₂ needs to be foamed. Furthermore, the nanoclay mayimprove the barrier properties (low diffusion coefficient for both massand heat), insulation properties (low heat conductivity), mechanicalproperties, and heat resistance, offering new opportunities in variousapplications.

Polystyrene/clay and PMMA/clay nanocomposites were prepared and used tomake nanocomposite foams. It was found that the cell size is greatlyreduced, and the cell density is increased, by adding a small amount ofclay. The clay dispersion also has a great influence on the cellmorphology. The exfoliated nanocomposite foam provides the highestdensity and lowest cell size. For exfoliated nanocomposite foams, ahigher clay concentration seems mainly to improve cell density. Addingclay not only provides sites for nucleation, but also changes therheological properties of the polymer matrix, which is also important infoaming process.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiment(s), but on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims, which are incorporated hereinby reference.

REFERENCES

The following references are hereby incorporated by reference:

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What is claimed is:
 1. A polymeric nanocomposite foam produced inaccordance with the method comprising the steps of: providing a mixturecomprising: a polymer, an organophilic clay, and a blowing agent; andprocessing said mixture so as to cause formation of cells, therebyforming a polymeric nanocomposite foam.
 2. A polymeric nanocompositefoam, said polymeric nanocomposite foam comprising: a polymeric portion;an organophilic clay, said organophilic clay dispersed throughout saidpolymeric portion; and a plurality of cells dispersed throughout saidpolymeric portion.
 3. The polymeric nanocomposite foam according toclaim 2 wherein said polymeric portion comprises a polymer selected fromthe group consisting of polystyrene, poly(methyl methacrylate),polypropylene, nylon, polyurethane, elastomers, and mixtures thereof. 4.The polymeric nanocomposite according to claim 2 wherein saidorganophilic clay is dispersed throughout said polymeric portion suchthat an x-ray diffraction pattern produced from said polymericnanocomposite foam is substantially devoid of an intercalation peak. 5.The polymeric nanocomposite according to claim 2 wherein saidorganophilic clay is dispersed throughout said polymeric portion suchthat a x-ray diffraction pattern produced from said polymericnanocomposite foam contains an intercalation peak.
 6. The polymericnanocomposite according to claim 2 wherein said organophilic clayportion comprises: a smectite clay; and a compound having the formula:

wherein: R1 is (CH)_(n) wherein n ranges from 6 to 20; R2 is a chemicalstructure having a terminal reactive double bond; R3 is an alkyl group;and R4 is an alkyl group.
 7. The polymeric nanocomposite according toclaim 6 wherein n is 15, R3 is CH₃, R₄ is CH₃, and R2 is:


8. The polymeric nanocomposite according to claim 6 wherein saidsmectite clay is selected from the group consisting of montmorillonite,hectorite, saponite, laponite, florohectorite, and beidellite.
 9. Thepolymeric nanocomposite according to claim 2 wherein said polymericnanocomposite foam has an average cell size less than about 20 microns.10. The polymeric nanocomposite according to claim 2 wherein saidpolymeric nanocomposite foam has an average cell size greater than about15 microns.
 11. The polymeric nanocomposite according to claim 2 whereinsaid polymeric nanocomposite foam has an average cell density greaterthan about 1×10⁶ cells/cm³.
 12. The polymeric nanocomposite according toclaim 2 wherein said polymeric nanocomposite foam has an average celldensity greater than about 1×10⁹ cells/cm³.
 13. The polymericnanocomposite according to claim 2 wherein said polymeric nanocompositefoam is closed cell foam.
 14. The polymeric nanocomposite according toclaim 2 wherein said polymeric nanocomposite foam is open cell foam.